This invention relates generally to ion generators, and more specifically, to improvements that provide extended period operation at higher beam currents in a negative ion generator of the modified calutron type wherein negative ions are produced by surface ionization through the bombardment of a converter plate with positive ions of selected species produced in an electron arc discharge column.
In the art of negative ion production, one source of ion production is a modified calutron source employing the concept of surface ionization. One particular source of this type is the SITEX (Surface Ionization with Transverse Extraction) ion source. The SITEX ion source utilizes a hot filament arc discharge in a magnetic field to improve the gas efficiency of negative ion generation. The electron stream coming from the hot filament covers a larger area than is needed for generating the positive ions from selected molecular species, such as hydrogen or deuterium and cesium. The uncollimated electron power is dissipated on a graphite arc anode collimator, causing it to heat up to sublimation temperature. This, in turn, causes the collimator slot to erode very quickly and the ion source to fail prematurely. An additional disadvantage is that large quantities of carbon impurities are deposited on a cesium-coated, molybdenum ion converter plate, thereby raising the surface work function for the conversion of the positive ions to negative ions, and thus lowering the negative ion production efficiency.
In order to increase the ion source operating time, particularly to operate the source at steady state rather than in a pulsed mode, attempts were made to solve the problems associated with the graphite arc collimator. One attempt included designing a water-cooled molybdenum anode collimator. However, this has not been used, since during extended pulse periods the electron discharge power density .about.6KW/cm.sup.2) is too high for this arrangement to handle and the molybdenum collimator would melt during very short periods of operation. Thus, it will be appreciated that there is a need for an electron discharge collimator for use with the SITEX ion source which will allow the ion source to run short pulse to steady state.